1. Technical Field
The invention relates to brake lining wear sensing and brake lining transient temperature detection and more particularly to a modular electrical resistance sensor which may be used to implement wear and temperature sensing.
2. Description of the Problem
Effective brakes are essential to safe motorvehicle operation. Contemporary brake systems dissipate vehicle kinetic energy through brake friction pads as heat. These brake pads have a relatively short service life and require regular replacement. Heavy vehicles have historically exhibited problems with brake overheating, especially when the vehicles are descending along long grades. Overheating reduces stopping ability and accelerates brake pad wear.
Inspection of the brake system has traditionally involved disassembly of the wheel mechanism and visual examination of the pads. It has been recognized that it would be desirable to incorporate some kind of sensor into the brake pads that monitor wear of the pads without the need to disassemble the brake system. Were the same sensor used to monitor brake temperature the addition to vehicle complexity would be minimized.
Various brake lining wear detection systems and brake temperature measurement systems are known in the art. One such system for detecting wear provides a modular, progressive brake lining wear sensor. The sensor has a triangular, wedged shaped electrically resistive member disposed between a pair of conductive plates to define a triangular shaped sensor. The sensor is encapsulated within an erodable molding and connected to a sensing circuit by a pair of leads including a ground lead and a resistive lead. The ground lead and resistance lead emerge from the encapsulated sensor for connection to the sensing circuit. The sensor is disposed within the brake lining and is connected to the brake shoe. As the brake lining progressively wears, the triangular wedged shaped resistive member is also progressively worn away thus continuously changing the overall resistance of the sensor. The change in resistance provides for continuous indication of the state of wear of the brake lining.
Another sensor design provides both wear and temperature sensing. Here a plurality of parallel connected resistors are connected to a sensing circuit. A thermistor provides temperature sensing. The resistors and the separate thermistor are mounted, spaced from one another, on a printed circuit board and the entire unit encapsulated within a single molding. The thermistor is connected to a grounded lead as are each of the resistors. A ground lead, a resistance lead and a thermistor lead emerge from the encapsulated module for connection to the sensing circuit. The module is disposed between linings in a drum brake so that the module is worn away with the linings. With progressive wear the resistors (or at least the conductive loops in which individual resistors are connected) are progressively and sequentially worn away, increasing the resistance of the sensor in a series of discrete steps. Three resistors are used to indicate 4 discrete levels of brake lining wear.